Zoom lenses used in television cameras for broadcasting are expected to realize high performance over the entire zooming range, the entire focusing range, and over the entire image plane, as well as to satisfy certain conditions associated with the use of a solid state image pickup device, such as a CCD. In addition, there is an increasing demand for zoom lenses with higher magnifications and larger zoom ratios.
One of the prior art zoom lenses that fulfills the above requirements is disclosed in Japanese Laid-Open Patent Application 2001-183584. The zoom lens of this Japanese patent application consists of four lens groups, namely, a focusing lens group, a variator lens group, a compensator lens group, and a relay lens group in order from the object side, and achieves a zoom ratio of 66.
It is difficult to achieve high optical performance over the entire range of zoom and range of focus with a zoom ratio of 80 or larger and with large aperture ratios, for example with the f-numbers being between 1.8 and 3.3. In particular, the refractive indices of the lens elements of the lens groups, the lens element configurations of the lens groups, as well as the chromatic properties for achromatization, must be properly and strictly prescribed in order to achieve high optical performance without increasing the size or weight of the lens system too much, that is, without excessively increasing the spacings between the lens groups or the number of lens elements.
Aberrations at the telephoto end of the range of zoom and fluctuations in aberrations associated with focusing need to be minimized in current applications for high definition broadcasting systems such as high definition television systems. More particularly, it is difficult to achieve high resolution without reducing the absolute value of the lateral color and without reducing fluctuations in the lateral color throughout the entire range of zoom and the range of focus.